In recent years there has occurred a combining of the magnetic tape recording and optical disc recording arts resulting in the evolution of high density recording on light and thermal sensitive optical tapes using techniques analogous to helical scan magnetic tape recordings. Optical tape media using magneto-optic and reversible crystalline state techniques for erasable recordings, as well as ablative techniques for permanent or archival recordings, are in current use and development.
The combination of the two arts has derived the benefit of the extremely high data packing density and, in the case of ablative recording, permanent storage of data information from the optical recording art, and the extensive recording surface and increased portability of the recording system and storage media from the magnetic tape recording art. An optical tape recording system is particularly well suited for tactical applications where information is to be recorded at field sites and the storage media returned to a base station for playback and processing of the recorded information.
Multiple channel recording and playback techniques are used in both magnetic tape and optical disc systems for greatly increased data transfer rate capability. Therefore, it is apparent that this advantage would apply also to optical tape recording and playback systems. In optical storage systems there are typically two different methods of forming the multiple beams used for recording and playback. In one method, a single laser produces a beam which is split into a plurality of beams by, for example, an optical diffraction grating or an acousto-optic beam splitter. The acousto-optic device is particularly well suited for recording, as the beams may be individually modulated by the beam splitter. In the second method, an array of semiconductor laser diodes produces a plurality of beams which may be driven by independent sources.
An optical tape recording headwheel may ideally use a laser diode array for generating record beams. A typical record headwheel, having a five-inch diameter, will accommodate an array of five or more laser diodes, collector lens, beam shaping optical elements, objective lens and focus subsystem. Thus, the headwheel is relatively lightweight and has low power requirements. However, since the diode array occupies a significant fraction of the volume of the headwheel, there is not sufficient space to include the additional elements needed for playback within the record headwheel. Thus, a different headwheel for use in playing back the data recorded on an optical tape is proposed.
It is advantageous to use a beam produced by a gas or ion type continuous wave laser for playing back data recorded on an optical tape, as the smaller wavelength beam from an argon or helium-neon laser can be focused to a smaller spot than can the beam from a semiconductor laser. In this way the tracks of recorded data can be placed much closer together and the packing density increased, since the shorter wavelength of the playback laser beam reduces cross-talk between tracks and inter-symbol interference along each track.
Because of its size, a gas or ion laser cannot be located on a rotating headwheel. It would be necessary therefore, to direct a laser beam from a stationary position onto the rotating playback headwheel such that a playback objective lens illuminates the optical tape forming a helix around the rotating headwheel. However, if multiple beams are generated from a stationary location onto the headwheel, the beams would rotate about the optical axis of the playback lens and would be unable to properly illuminate the data information recorded on the tape.